This invention relates to methods and apparatus for detection of phase-and-amplitude-shift-keyed communication signals that may experience multipath propagation and/or other dispersion as well as additive noise prior to reception. The terms detection and demodulation are used synonomously throughout. The method applies to electromagnetic, acoustic, and other types of communication signals that travel through the atmosphere, water, or any other medium. As does all known prior art, it requires that the signal have a block format with a fixed, known pattern interspersed between data segments.
The problem that inspired the concept and development of the invention is that of detecting a quadraphse-shift-keyed (QPSK) signal transmitted through a fading channel. Of primary interest is the high-frequency (HF) radio band that is heavily used for long-haul commercial and military communications of many types. It is well known that such signals propagate for long distances due to one or more reflections from the ionosphere and the surface of the earth and that the fading of these signals often experienced at the receiver is due largely to the interference that occurs when there are multiple paths by which the signal reaches the receiver (cf. Reference 1). The unreliability of communication due to fading has classically been dealt with by various diversity techniques discussed in Reference 1.
In the last decade or so, substantial effort has been devoted to the improvement of HF detection directly without using diversity techniques (cf. References 2-5). There are many reasons for this: (1) In some applications diversity techniques are not feasible. (2) Watterson et al (Reference 6) have constructed a detailed mathematical model of HF propagation that has been quite widely accepted as satisfactory for the design and testing of HF communication systems. (3) Automatic equaliztion techniques had been sucessfully used on telephone channels (cf. Reference 7). (4) Theory and practice of adaptive systems were growing rapidly. (5) Relatively inexpensive microprocessors had become available to implement much more powerful algorithms in real-time. (6) It seemed that the performance attained on dynamic fading channels was quite far from its limit (cf. Reference 8).
Probably due to the influence of Reference 7 all recent approaches to improved detection at HF use adaptive equalization wherein intersymbol interference is minimized by setting tapweights of a transversal filter in response to a training signal. The weights are usually adjusted during the data portion of the transmission by a tracking algorithm. It has proved to be difficult to detect the data conveyed by the signal and to minimize the intersymbol interference simultaneously, because an error in one function tends to degrade the other function. Rapidly fading channels have been particularly difficult to handle satisfactorily by this approach (cf. Reference 4).
The approach used by the invention disclosed here is quite classical, but substantially different from the popular decision-feedback equalization used by others. Basically it computes an estimate of the multipath-structure of the channel that prevails for each block of data, then it uses this estimate to coagulate the dispersive effects of the channel so that data detection can be performed by ordinary means.
The invention avoids the convolutions that time domain computations would require by working in the frequency domain via the fast Fourier transform (FFT).
Performance of the invention is substantially improved by testing sets of symbols likely to be in error using a metric that measures the discrepancy between observation and the implication of the estimates.
The invention disclosed here has applications not only to electromagnetic signals at HF but also in other frequency bands and to accoustic signals where multipath propagation and/or dispersion can degrade performance.